1. Field of the Invention
The present invention relates to an optical deflector for deflecting and sweeping a light beam such as a laser beam.
2. Description of the Related Art
Recently, as a MEMS (micro electro mechanical systems) device using semiconductor processes and the micro machining technology, an optical deflector has been proposed in which a mirror (a reflecting board) and electromechanical parts such as movable parts for the rotary drive of the mirror are formed integrally on a semiconductor substrate. In the optical deflector, in order to improve the deflection and sweeping performance further, it is desired that the drive status (e.g., the deflection angle and the deflection velocity) is controlled fast and precisely. For that purpose, the feedback control is required by detecting the drive status (e.g., a rotation angle) of the mirror.
As a method to detect the rotation angle of the mirror, an example is a technique in which laser light deflected/swept by the mirror is detected by a separately provided sensor. However, in this technique, the system size is increased. To avoid this problem, techniques have been proposed in which an optical deflector itself carries a detection mechanism (See e.g., Patent Documents 1-3).
In the optical deflector in Patent Document 1, an end of a movable frame part is connected to and supported by a stationary part, and a torque, which is generated in the movable frame part, is transmitted to a torsion bar (an elastic beam) which is connected to the other end of the movable frame part, to rotarily drive a mirror which is installed at the end of the torsion bar. A piezoresistive element is built in the anchor part of the elastic beam on the side of the frame to detect the twist angle of the elastic beam, thereby detecting the rotation angle of the mirror. The optical deflector in Patent Document 2 utilizes an electromagnetic drive method in which a sensor for detecting the variation of the magnetic field is installed to detect the angle of the mirror that has a thin film of hard magnet. The optical deflector in Patent Document 3 utilizes an electrostatic drive method in which the electrostatic capacitance between the mirror and an electrode facing the mirror is measured to detect the angle.
(Patent Document 1)
Japanese Laid-open Patent Application, Application No. 2004-226548
(Patent Document 2)
Japanese Laid-open Patent Application, Application No. 2000-235152
(Patent Document 3)
Japanese Laid-open Patent Application, Application No. 2006-184603
(Patent Document 4)
Japanese Laid-open Patent Application, Application No. 2005-128147
On the other hand, an optical deflector that uses a piezoelectric drive method using piezoelectric actuators as the driving source of the mirror has been proposed (See, e.g., Patent Document 4). In the optical deflector, one end of the piezoelectric actuator is connected to and supported by the frame part (the support), and a torque, which is generated by the piezoelectric actuator, is transmitted to the other end of the torsion bar (the elastic beam) to rotarily drive the mirror that is installed at the end of the torsion bar. Advantages of this optical deflector are: it is small in size; it can provide a large driving force with a simple structure; and it is easy to be manufactured and mass-produced.
In this optical deflector using piezoelectric actuators, in order to detect the rotation angle of the mirror, one may consider utilizing the technique disclosed in Patent Document 1: that is, the rotation angle of the mirror is detected by detecting the twist angle of the elastic beam by a piezoresistance element or a strain gauge installed at the anchor end part (on the side of the support) of the torsion bar which supports the mirror. However, due to the built-in piezoresistance elements or strain gauges, a complex structure would result.
On the other hand, instead of the piezoresistant element, one may consider installing a separate piezoelectric sensor to detect the angle displacement by the piezoelectric electromotive force, or installing additional wiring for a sensor to the piezoelectric actuator besides the wiring for driving the piezoelectric actuator. In this case, the detection mechanism can be formed by using the manufacturing process of the piezoelectric actuator for driving the mirror.
However, because the twist displacement of the torsion bar is minute, it is difficult to measure the twist displacement with a good S/N ratio. Furthermore, because the position of the piezoelectric sensor is close to the piezoelectric actuator, a crosstalk of the sensor signal with the driving signal of the piezoelectric signal is a problem.
Furthermore, recently, as MEMS (micro electro mechanical systems) devices that utilize a semiconductor process and/or a micro machining technology, optical deflectors (a micro optical scanner) in which mirrors and piezoelectric actuators are formed integrally on a semiconductor substrate have been proposed (See, e.g., Patent References 5-7). In these optical deflectors, an end of a piezoelectric actuator is connected to and supported by a frame part (a support), and a torque, which is generated in the piezoelectric actuator, is transmitted to a torsion bar (an elastic beam) which is connected to another end of the piezoelectric actuator, to rotarily drive a mirror attached to an end of the torsion bar.
Foreign Patent Document 5: Laid-open Patent Application 2001-179180
Foreign Patent Document 6: Laid-open Patent Application 2005-128147
Foreign Patent Document 7: Laid-open Patent Application 2005-148459
In these types of optical deflector, in order to improve the deflection/scanning performances, it is desirable to increase the maximum deflection angle and to control the deflection angle and the deflection velocity faster with greater precision. Further, in order to rotarily drive a mirror, these optical deflectors have moving parts (such as torsion bars and piezoelectric actuators) that may break or deteriorate when an excessive load is applied. Therefore, it is necessary to control the deflection angle and the deflection velocity within certain respective ranges such that the moving parts are not overloaded even when the moving parts are rotarily driven at its maximum allowed angle.
But, in the optical deflector of Patent Document 5, there exists a limit to the control performance of the deflection angle and the deflection velocity, because of the structure of the optical deflector by which the torque generated by the piezoelectric actuator is transmitted to the mirror through the torsion bar. Furthermore, the maximum deflection angle of the optical deflector is mostly determined by the torque generated by the piezoelectric actuator, the material and the shape of the torsion bar, and the shape and the weight of the mirror. Therefore, in order to obtain a larger deflection angle for the optical deflector of Patent Document 5 with same material and the same shape of the torsion bar and with the same shape and the same weight of the mirror, the torque generated by the piezoelectric actuator must be made larger. And in order to generate a larger torque by the piezoelectric actuator, one may consider increasing the size of the piezoelectric actuator, increasing the voltage applied to the piezoelectric actuator, and improving the property of the piezoelectric actuator,
But, increasing the size of the piezoelectric actuator leads to an increase in size of the overall device, resulting in a smaller number of chips per wafer. On the other hand, increasing the applied voltage to the piezoelectric actuator causes an increase in the power consumption of the device, and necessitates circuits that can operate with a higher operational voltage. Furthermore, it is technically difficult to improve the properties of the piezoelectric actuator due to limitations on the physical properties of piezoelectric materials.